The present invention relates to over-voltage arrestors, and, more particularly, to gas-tube arrestors incorporating a fail-safe mechanism to safely operate in the event of an over-voltage in a telecommunication line even when the gas tube is vented or at an over-temperature condition which thermally activates the fail-safe mechanism. These gas-tube arrestors are widely utilized to protect electronic equipment from an over-voltage condition caused by lighting, high-voltage line contact, and other similar circumstances.
Fail-safe gas-tubes are utilized to protect circuits from a sudden surge in the line voltage, caused by a lightning strike, or a continuous current flow at an elevated voltage from contact with a secondary power source, such as a power line. A conductor coupled between a source of current and a protectable circuit may have an arrestor coupled to it for actuation at an over-power condition for dissipation of a power surge to ground. One type of arrestor is a gas-filled tube coupled between the conductor and ground to divert or shunt the over-power in the conductor at a predetermined voltage. The gas in the tube is ionizable, and thus conductive at a predetermined voltage, to close the circuit to ground and shunt the over-power line surge. In addition, a continuous current flow through the gas-tube, as from inadvertent contact with a power line, may exceed the thermal capacity of the gas-tube leading to a fire hazard. Consequently, various thermally-activated apparatus with conductive elements have been melded with the gas-tubes to fuse or melt and solidly link the tube electrodes for continuous current flow to ground. Various fusible or meltable materials have been used to insulate the conductive elements, which insulative materials melt at temperatures correlative to power flow above a predetermined level.
Various fail-safe arrangements for electrical equipment have been provided with gas tubes and other types of protectors, such as air-gap arrestors. Exemplary of the considered problems and contingencies for the fail-safe arrangements are a sustained overload condition, such as contact with a power line, which would produce a continuous ionization of the gas tube from the passage of heavy current flows through the tube. Heavy current loads may, in fact, destroy the voltage protector, thus producing a fire hazard. A common approach to avoid this problem is to utilize a fusible element on the gas tube, which element fuses in the presence of overloads to permanently short circuit the arrestor and release another mechanism, such as a spring-loaded shorting bar, for coupling to ground and continuous protection from the overload. The permanent short-and-ground condition implies a condition requiring inspection and/or replacement of any damaged components as well as repair of the condition producing the continuous short. Examples of such fail-safe protection devices are illustrated in U.S. Pat. Nos. 3,254,179; 3,340,431; and 3,522,570.
The above-noted fusible elements are generally a metallic material, which forms an electrically conductive path to short circuit the arrestor or gas-tube electrodes. The particular choice of materials for such fusible component is dictated by the properties of the metal, including its thermal conductivity. An altenative structure in U.S. Pat. No. 4,212,047 to Napiorkowski utilizes a fusible material, which is an electrical insulator, interposed between one or more of the electrodes in a shorting mechanism. This apparatus is particularly directed to thermal conditions for direct short circuiting after fusing of the insulative material. Among the class of materials taught in this patent are certain fluoroplastics, including fluorinated ethylene propylene polymer ["FEP"]. These materials are solid components and provide no communication between the electrodes without complete fusion of the fusible member.
A gas-tube with a temperature-responsive device is provided in U.S. Pat. No. 4,717,902 to James, and utilizes a coated apparatus to short circuit an electrical conductor at an over-temperature condition. More specifically, a conductive element has a coating, such as a varnish, which decomposes at an elevated temperature and allows contact between electrodes of the shorting apparatus for conduction of the over-current or over-voltage condition to ground. The conducting members are illustrated and taught as spring-tempered components.
An excess voltage arrestor with a coated wire interwoven between several electrodes of an arrestor is shown in U.S. Pat. No. 4,371,911 to Baker. At a continous over-voltage or over current condition, the sleeve material surrounding a conductive element is melted sufficiently to provide the tensed, interwoven element to expand through the softened insulating material and contact the electrodes to provide a continuous short-circuit. Similar meltable materials on wound wire elements are illustrated in U.S. Pat. No. 2,992,310 to Babany and U.S. Pat. No. 3,046,536 to Sciuto. In these applications, the apparatus are utilized for fire alarm energizing apparatus with a thermo-plastic wrap or insulating material on the wiring, which wrap is meltable at a specific or predetermined temperature.
Gas-tube arrestors may be inadvertently vented, which effectively relegated them to air-gap insulators. Therefore, it has been found desirable to provide an assembly providing continuous conduction across the electrodes at a thermal overload, as well as providing an ionizable chamber to conduct during small line surges, and further providing conduction through the fail-safe mechanism in the event of inadvertent venting of the gas-tube enclosure or body.